1. Field of the Invention
The present invention relates to a polymer latex that is particularly suitable for the production of dip-molded articles. Furthermore, the present invention relates to a process for making such a polymer latex, to the use of said polymer latex for the production of dip-molded articles, to a compounded polymer latex composition that is suitable for the production of dip-molded articles, to a method for making dip-molded latex articles, as well as to the latex articles obtained thereby.
2. Description of the Related Art
Thin wall extensible articles, such as gloves and other medical products have long been made from a natural latex polymer. Typically such articles are formed from natural rubber latex, naturally occurring emulsion of polymer and water with added stabilizing agent and vulcanizing chemicals. Additionally, in order to achieve the desired mechanical properties products made from natural rubber latex are vulcanized using a sulfur-base vulcanization system that also includes accelerators.
Such sulfur-based cure systems have been also employed for vulcanizing synthetic polymer lattices thereby introducing sulfur-based links during the crosslinking of the polymer composition. In these sulfur-based cure systems in addition to sulfur accelerators, such as amines, thiazoles, sulphenamides, dithiocarbamates and thiuram are utilized.
EP-A-1 063 258 relates to a latex composition for dip molding wherein C8-22 fatty acids or salts thereof are incorporated as bubble breakers. This latex composition is vulcanized using additional sulfur based vulcanization systems in order to produce the dip molded products.
Similarly EP-A-559 150 discloses a copolymer latex suitable for dip molding whereby the latex is vulcanized by a traditional sulfur vulcanization system.
EP-A-753 530 relates to a different technology. Herein a polymer latex is described having segments of different glass transition temperature. This polymer latex is used for making mattresses whereby traditional sulfur containing vulcanization systems are used. This application does not relate to dip molding.
Recently it has been discovered that latex articles made either of natural rubber lattices or of synthetic rubber lattices using sulfur-based cure systems may induce allergenic reactions of the type IV.
Since one important field of application for natural and synthetic rubber articles is, for example, medical gloves or condoms that inevitably will come in contact with the skin of the wearer and due to the increasing susceptibility to allergenic reactions of people in modern society it has been a long-felt need in industry to provide synthetic rubber compositions that can be formed into dip-molded articles, like medical gloves, that do not induce allergenic reactions or liberate compounds that may induce other health risks, like for example nitrosamine without compromising the desired and necessary properties of these products, like mechanical strength, elasticity. Furthermore, it is especially or medical gloves desired that they possess adequate esthetic properties with respect to drape, softness and tactility. Furthermore it is most important for these products that they provide a good barrier to microbial penetration and are substantially impermeable to a variety of liquids and gases. Therefore, in addition to the desired mechanical properties it is also important that the final product has a uniform film thickness.
In the prior art several approaches to substitute a sulfur-based curing system containing accelerators were discussed in order to avoid allergenic reactions caused by the rubber articles.
WO 00/11980 describes synthetic rubber lattices and aqueous polyurethane dispersions having very low glass transition temperatures that are crosslinked by means other than sulfur vulcanization of double bonds. Particularly the synthetic rubber should be substantially free of carbon-carbon unsaturation. Thus no diene component shall be used. However, diene monomers can be used as long as sulfur vulcanization is not applied for crosslinking the resultant rubber. The polymers described therein are capable to be crosslinked using external crosslinkers. Suitable crosslinking functionalities in the polymers are hydroxyl or carboxyl groups. But nevertheless external crosslinkers are necessary that have a potential not to be bound to the polymer and therefore to bloom out to the surface of the polymer in the final product, and therefore in itself may cause health risks, especially in view of the selections of crosslinking agents disclosed in that reference.
An alternative solution to avoid sulfur-based vulcanization systems containing also accelerators has been described in WO 02/50177. Herein metal oxide cros slinking agents are used for cros slinking the synthetic polymers. Suitable examples are zinc oxide, magnesium oxide or cadmium oxide.
A similar solution has been suggested in WO 02/38640 disclosing rubber compositions containing chelating monomers that can be crosslinked with polyvalent metal ions. A suitable chelating monomer is an acetoacetoxy functionalized monomer.
EP-A-1 361 217 discloses a polymer latex composition for dip molding comprising a carboxylated conjugated diene based rubber latex having according to the examples a single glass transition temperature whereby crosslinking is achieved by the presence of multivalent cations.
WO 03/006513 discloses latex formulations comprising a base polymer having carboxylate groups, a divalent or trivalent metal, an amine or amino compound and a neutralizing agent in an amount sufficient to neutralize at least a portion of the carboxylate groups in the base polymer.
WO 03/062307 discloses a polymer latex composition. The polymer particles thereof can be made of different segments having different glass transition temperatures. These compositions are either used as coating composition or as a component of a coagulant for a dip molding process. But in both cases this composition only forms a coating and the bulk material of the dip molded article is a conventional sulfur vulcanized polymer latex. It is essential to the invention described in WO 03/062307 that the polymer latex having segments of different glass transition temperature is stabilized using polyvinyl alcohol. Such systems are extremely stable against electrolytes which also can be seen from the embodiments where this latex is used in combination with a coagulant. Even in this coagulation composition the latex remains stable which is a clear indication that the latex has extremely high resistance to coagulation due to the presence of polyvinyl alcohol as stabilizer. For the bulk material of the dip molded articles a conventional sulfur cured polymer latex is used. The objective solved by the approach disclosed in WO 03/062307 is to reduce the stickiness of the inner and outer surface of the glove. Therefore it is also only necessary to use the particular polymer latex described in WO 03/062307 as a coating whereas the bulk material of the dip molded article is still a conventional sulfur vulcanized polymer latex. Therefore also the latex used for the coating does not need to fulfill the tensile strength requirement for dip molded articles.
Other solutions to avoid sulfur-based vulcanization systems containing accelerators for crosslinking synthetic or natural rubber lattices in order to produce health care products still employ additional cros slinking agents. These are either organic molecules having a functionality adapted to react with the cros slinking functionality in the rubber component or polyvalent metal salts. Thus the rubber compositions still have to be compounded with the crosslinking agent which results in a complicated process for making the latex compound. In this process the relative amounts have to be carefully adjusted in order to achieve the desired crosslinking density, and if possible to bind all the additional crosslinkers into the final molecule to avoid blooming out of crosslinker component that in itself again may create a health risk. Furthermore, especially when using polyvalent metal ions as crosslinking agent latex instability during the compounding process may occur which makes the compounding process particularly difficult. Furthermore the crosslinking agents, especially the polyvalent ions, reduce the stability of the latex compound and therefore the ability to store the already compounded latex composition prior to its use in a dip-molding process for making the medical care product. Furthermore, especially the introduction of polyvalent metal ions as crosslinking system in rubber compositions may increase environmental hazards.
WO 02/18490 discloses a different approach wherein a hydrogenated tri-block copolymer containing, for example, styrene-ethylene/propylene-styrene segments which do not contain double bonds, is mixed with a cyclic unsaturated polyolefin crystalline polymer. This polymer composition is not a latex and the articles made of this composition have to be extruded. Furthermore still crosslinking is necessary to obtain the desired mechanical properties of the final product. Crosslinking can be achieved either by physical means, like radiation, or by chemical means, like peroxides and usual vulcanization systems, as disclosed above.
From WO 01/30876 substitutes for natural or synthetic rubber lattices are disclosed that can be used to make medical care products. Thermoplastic elastomeric tri-block polymers comprising two polystyrene hard domains and one polyolefin rubber domain are prepared by using living anionic polymerization. The resultant polymer, due to the use of butadiene or isoprene, in the polyolefin rubber domain, still have double bonds available for crosslinking. Typical crosslinking processes disclosed in WO 01/30876 are radiation or chemical vulcanization processes using well-known sulfur/accelerator systems.
Consequently the elastomeric polymers to be used or making medical care products according to the teaching of both latter prior art documents have the disadvantage that still crosslinking is necessary. Either a complex radiation curing has to be employed which in itself is a health hazard, or traditional vulcanization systems that suffer from the drawbacks as discussed above, have to be applied. Finally conventional dip-molding processes cannot be applied using these polymer systems in order to make the desired products.
U.S. Pat. No. 5,500,469 describes a thermally gellable artificial latex composition useful for preparing articles such as gloves, condoms or balloons that are free of vulcanizing agents and proteins. The polymer of the artificial latex is a preformed multiblock copolymer prepared by anionic polymerization in solvents. Due to the anionic polymerization process the number of monomers which can be used for block copolymerization is very limited. U.S. Pat. No. 5,500,469 describes only multiblocks consisting of a combination of type A block consisting of alkenyl aromatic hydrocarbons and type B block consisting of a conjugated diene. The block copolymers are dissolved in non polar hydrocarbon solvents up to a total solids of 20 to 50% of the organic phase. After adding a sulfate of an ethoxylated phenol as surfactant and water the hydrocarbon has to be completely removed by distillation. This means high volumes of organic solvents in relation to the polymer weight have to be handled. The limited versatility in respect of suitable monomers for the anionic block copolymerization process is another disadvantage of the teaching in U.S. Pat. No. 5,500,469.
In other fields of technology that do not relate to dip molded articles polymer latex lattices having segments of different Tg's are known. These lattices can, for example, be used as coating material whereby in these applications coagulation should be avoided under any circumstances since this would be detrimental to the desired coating properties.
For example, U.S. Pat. No. 5,872,189 discloses water-redispersible powders based on a polymer latex having “core/shell” structure with different glass transition temperatures. The key point of this invention is the redispersibility in water which can only be achieved if the latex is stabilized against coagulation. Consequently such a latex cannot be used in dip molding, and it is explicitly disclosed that these redispersible powders can be suitably used in the building industry as additives for hydraulic mineral binders for the production of protective and decorative coatings and of adhesive mortars and adhesive cements. Consequently vulcanization of these systems is no issue at all.
Similarly JP-A-2002-226508 relates to a paper coating composition wherein also electrolyte stability is of uppermost importance since otherwise no stable coating composition for paper coating can be obtained. Furthermore, like in U.S. Pat. No. 5,872,189 vulcanization of the latex is no issue for a latex used in paper coating compositions.